Electroplating baths and process for producing bright zinc deposits



United States Patent "as ch Int. Cl. C23b /12, 5/46 U.S. Cl. 2 0455 21 Claims ABSTRACT OF THE DISCLOSURE Semi-bright to fully bright zinc deposits are produced by electroplating articles in an acid electroplating bath containing a non-complex zinc salt, if required, a conductivity increasing salt, and, as brightening and the throwing power increasing additives, a primary amine or polyamine, especially an aliphatic or aromatic amine having at least four carbon atoms in its molecule, and a water soluble condensation product of a naphthalene sulfonic acid and formaldehyde. Dimethyl formamide may be added to the baths to prevent formation of precipitates therein.

Electroplating with such baths is preferably effected at a pH between about 2.5 and about 5.5, a temperature between about C. and about 40 C., and a current density between about 0.5 amp./dm. and about 5 amp./ dmf The baths are able to replace the heretofore used alkaline electroplating zinc baths and are substantially free of the disadvantages of said baths, especially the difficulties encountered in waste disposal.

The present invention relates to an acid galvanic electroplating zinc solution or bath and more particularly to an acid zinc bath for depositing semi-bright or fully bright zinc plates or coatings, to a elecroplating process by means of such an acid zinc bath, and to zinc coated artices obtained by electroplating in such a bath.

The known and presently used bright electroplating zinc baths operate exclusively in the alkaline range. EX- tensively used are bright electroplating zinc baths based on cyanide. However, such cyanide baths have the disadvantage that the used solutions obtained on electroplating must be completely detoxified by destroying their remaining cyanide content to an amount of 0.1 mg./l. of sodium cyanide before they can be discharged into the sewer. Such detoxification is rather expensive so that the economy of the electroplating process when using such cyanide baths is unfavorably affected.

For this reason cyanide-free alkaline zinc baths have been developed. In such baths the zinc is present in the form of a zincate. However, the use of zincate alone permits only the preperation of baths of moderate zinc concentrations so that, as a result thereof, only comparatively low deposition current densities can be achieved. To increase the zinc concentration in such baths and thus the current density, it has been suggested to add to the cyanide-free alkaline zincate baths other non-toxic complex-forming agents. Such complex-forming agents, however, in contrast to cyanide, are not destroyed when Working up the used solutions. Thus they pass over into the neutralizing plant wherein they prevent removal of zinc, and also of other metals, from the waste Water to an extent as required by law.

Acid zinc baths have technically been used for special purposes. The waste solutions remaining after electro- Patented Nov. 3, 1970 ploting with such acid zinc baths can be treated without difficulty in the neutralization step in such a manner that the zinc content of the used solutions to be discharged meets the legal requirements with respect to their disposal. However, the heretofore known technically useful acid zinc baths have the disadvantage that matte zinc deposits are obtained, on the one hand, while, on the other hand, the throwing power is much lower than that of the known alkaline zinc baths.

Many suggestions have been made to provide acid bright electroplating zinc baths. If bright deposits can be produced, the current density range at which such bright deposits are obtained, is rather limited. The throwing power of such baths is also quite low. Thus'the narrow range of current densities and the low throwing power of the heretofore proposed acid bright-electroplating zinc baths rendered such baths unsuitable for general technical applications.

It has olso been suggested to add triethylamine as a brightener to a zinc sulfate bath. However, such a bath requries that the zinc must be deposited on a rotating cathode. Furthermore, the high current densities must be kept strictly constant during electroplating. It is evident that such a bath cannot be generally employed for electroplating. For this purpose a wide range of current densities as well as conventional electroplating conditions are indispensable for depositing bright zinc plates or layers.

It is an important object of the present invention to provide an acid electroplating zinc bath which yields not only fully bright zinc deposits, i.e. plates over a Wide range of current densities but the throwing power of Ivavhilch approximates that of alkaline electroplating zinc at s.

Another object of the present invention is to provide a simple and effective electroplating process whereby bright to fully bright zinc deposits are obtained under economic conditions.

Still another object of the present invention are zinc electroplated articles of bright appearance which articles are obtained by electroplating in an acid electroplating bath of a specific composition to produce bright zinc deposlts.

Other objects of the present invention and advantageous features thereof 'will become apparent as the description proceeds.

In principle the acid electroplating bath for depositing semi-bright to fully bright zinc plates according to the present invention comprises an aqueous solution of zinc salts, especially of simple, i.e. not complex inorganic zinc salts and preferably of zinc sulfate, zinc chloride, or zinc fluoborate, and others, which may contain, if required, the conductivity improving agents, i.e. so-called conducting salts. In addition to these components the bath according to the present invention contains:

(a) primary amines or polymamines which contain at least four carbon atoms in their molecule and (b) water soluble condensation products of naphthalene sulfonic acids with formaldehyde.

The desired efiect, i.e. the formation of bright zinc deposits within a wide range of current densities and with a technically satisfactory throwing power is achieved only when adding to the acid zinc bath not only the primary amines but also the condensation products of naphthalene sulfonic acids with formaldehyde.

Useful primary amines or polyamines are aliphatic as well as aromatic amines. Preferred amines of this type are, for instance, n-amylamine, n-hexylamine, diethylene triamine, triethylene tetramine, aniline, pand o-toluidine, pand m-amino phenol, oand p-anisidine, and others.

The amines start to exhibit their brightening effect when added to the bath in amounts of about 0.5 g./ 1. to about 1.0 g./l. bath. When using amines of a lower molecular weight, usually larger amounts are required than with amines of a higher molecular weight in order to deposit fully bright zinc plates.

Although aliphatic amines with more than 8 to 10 carbon atoms produce also bright zinc deposits, such zinc plates are usually not as ductile as it is desired for technical purposes. Thus such higher amines are preferably used if the ductility of the zinc layer is of no particular importance.

The primary amines and polyamines are preferably added to the electroplating zinc bath in the form of their acid addition salts such as their hydrochlorides, sulfates and the like. When adding said amines in the form of their free bases, a corresponding amount of an acid, preferably a mineral acid is to be added to the bath.

The second brightening component is the condensation product of a naphthalene sulfonic acid with formaldehyde containing two or more naphthalene monoor disulfonic acids Which are connected with each other by means of methylene bridges. These condensation products are produced according to known methods as they are described, for instance, in Houben-Weyl Methoden der organischen Chemie, vol. XIV/2, page 316; in German Pat. No. 292,531, and in other literature references. Depending upon the type of primary amine or polyamine added, the condensation products are added in amounts between about 0.2 g./l. and about 20 g./l. Higher concentrations, although not required, are usually not harmful.

The acid zinc electroplating baths according to the present invention are preferably operated at a temperature between about 10 C. and about 40 C. at a pH between about 2.5 and about 5.5. Bright zinc deposits are also obtained at a lower pH-value. However, the current yield or current efficiency gradually decreases under such con ditions. A further disadvantage is that the anodes start to dissolve even if no current is passed through the bath. As a result thereof the zinc concentration in the bath increase continually. This is not desirable on continuous electroplating operation. At a higher pH-value than about 5.5, basic zinc salts precipitate in the electrolyte.

When adding aliphatic amines, the electroplating bath usually remains colorless even on prolonged operation.

However, when adding aromatic amines to the electroplating bath, brownish discoloration of the electrolyte is usually observed after operating the bath for some time. Thereby, there may readily be formed precipitates in the baths at an electrolyte temperature of 20 C. and lower and a pH-value exceeding a pH of 4.0. Said precipitates, although they have no disadvantageous effect upon the deposition of zinc, nevertheless are highly undesirable. They can be avoided by adding dimethyl formamide to the baths. Amounts of ml./l. to 100 ml./l. bath of dimethyl formamide are required depending upon the aromatic amine added.

Bright zinc deposits or plates obtained by using electroplating baths according to the present invention can be subjected to a postplating treatment whereby they are provided with chromate conversion coatings which convert the zinc surface from the naturally occurring oxide to the one containing hexavalent chromium. Chromate conversion coatings may be colorless or may impart various colors to the zinc deposits such as yellow, brown, or blive. The brightness of the zinc deposit is even enhanced especially by colorless or bluish chromate treatment.

The following examples serve to illustrate bright electroplating zinc baths according to the present invention. Semibright to fully bright zinc coatings are deposited from such baths at current densities between about 0.5 amp./ sq. dm. and about 5 amp./ sq. dm. and a bath temperature between about 15 C. and about 30 C. The baths have a throwing power which is good enough to employ them for zinc plating of technical articles in place of alkaline zinc electroplating baths.

4- EXAMPLE 1 One liter of electroplating bath contains:

300 g. of zinc sulfate,

20 g. of ammonium chloride,

40 g. of diethylene triamine, and

2 g. of the condensation product of naphthalene sulfonic acids and formaldehyde.

pH of the bath: 3.5.

EXAMPLE 2 One liter of electroplating bath contains:

g. of zinc sulfate,

60 g. of zinc chloride,

18 g. of triethylene tetramine, and

5 g. of the condensation products of naphthalene sulfonic acids and formaldehyde. 3

pH of the bath: 4.2.

EXAMPLE 3 One liter of electroplating bath contains:

g. of zinc acetate,

30 g. sodium sulfate,

8 g. of aniline hydrochloride,

25 g. of dimethyl formamide,

0.6 g. of the condensation product of naphthalene sulfonic acid and formaldehyde.

pH of the bath: 4.7.

EXAMPLE 4 One liter of electroplating bath contains:

300 g. of zinc sulfate,

40 g. of sodium chloride,

8.5 g. of o-toluidine hydrochloride, and

3 g. of the condensation product of naphthalene sulfonic acid and formaldehyde.

pH of the bath: 3.0.

EXAMPLE 5 One liter of electroplating bath contains:

250 g. of zinc sulfate, 30 g. of ammonium sulfate, 4.5 g. of p-anisidine, 30 g. of dimethyl formamide, and 8 g. of the condensation product of naphthalene sulfonic acid and formaldehyde. pH of the bath: 3.2.

EXAMPLE 6 One liter of electroplating bath contains:

275 g. of zinc sulfate,

25 g. of ammonium chloride,

15 g. of n-hexylamine hydrochloride,

15 g. of the condensation product of naphthalene sulfonic acid and formaldehyde.

pH of the bath: 3.0.

If the electroplating baths prepared according to the above mentioned examples do not have the indicated pH- value, hydrochloric acid added to adjust the baths to the indicated pH-value. This adjustment is especially required when the amines are used in the form of the bases and not in the form of their salts.

The electroplating baths according to the present invention yield fully bright deposits at the current densities and bath temperatures as given hereinabove. The throlwing power of the bath is highly satisfactory.

Of course, many changes and variations in the primary amines and polyamines and the condensation productsof naphthalene sulfonic acids and formaldehyde used, in the salts which serve to increase the conductivity of the baths, in the amounts of the bath components and additives thereto, in the pH-value of the baths, in the current densities and bath temperatures during electroplating, and the like may be made by those skilled in the art in accordance with the principles set forth therein and in the claims annexed hereto.

The preferred condensation products of naphthalene sulfonic acids and formaldehyde mentioned hereinabove are those prepared by condensing naphthalene monosulfonic acids with formaldehyde.

Two or three of these naphthalene monosulfonic acids are connected with each other by means of methylene bridges. The condensation products are added in the form of their neutral salts.

Using the sodium salt of methylene bis-(naphthalene sulfonic acid) which is commercially available, in place of the condensation product of naphthalene monosulfonic acid and formaldehyde, the same result is achieved.

The amounts of said salt or of the above mentioned condensation products are not dependent on the amounts of the amines added. An addition of 0.5 g. of said salt or said condensation products for each liter of electroplating bath is sufficient. However, preferably larger amounts are added.

The conducting salts mentioned hereinabove are alkali metal, ammonium, or magnesium sulfates or chlorides.

We claim:

1. An acid electroplating bath for depositing semibright to fully bright zinc plate, said bath comprising an aqueous solution of a zinc salt, a primary amine selected from the group consisting of a primary monoamine With at least four carbon atoms in its molecule and a polyamine with at least four carbon atoms in its molecule, and a water-soluble condensation product of a naphthalene sulfonic acid or salt thereof with formaldehyde, said condensation product containing at least two naphthalene sulfonic acid radicals connected with each other by methylene bridges, the primary amine being present in the bath in an amount of at least 0.5 g./l. and the water soluble reaction product being present in an amount of at least 0.2 g./l.

2. The acid electroplating bath according to claim 1 containing, in addition, a conductivity increasing salt selected from the group consisting of an alkali metal sulfate, ammonium sulfate, magnesium sulfate, an alkali metal chloride, ammonium chloride, and magnesium chloride in amount sufiicient to increase the conductivity of said bath.

3. The acid electroplating bath according to claim 2 containing, in addition, dimethyl formamide in an amount between about 5 mL/l. and about 100 rnl./ l.

4. The acid electroplating bath according to claim 1, wherein the primary amine is an aliphatic amine with at least four carbon atoms.

5. The acid electroplating bath according to claim 1, wherein the primary amine is an aliphatic polyamine with at least four carbon atoms.

6. The acid electroplating bath according to claim 1, wherein the primary amine is diethylene triamine.

7. The acid electroplating bath according to claim 1, wherein the primary amine is triethylene tetramine.

8. The acid electroplating bath according to claim 1, wherein the primary amine is aniline.

9. The acid electroplating bath according to claim 1, wherein the primary amine is o-toluidine.

10. The acid electroplating bath according to claim 1, wherein the primary amine is p-anisidine.

11. The acid electroplating bath according to claim 1, wherein the zinc salt is a non-complex zinc salt.

12. The acid electroplating bath according to claim 1, wherein the zinc salt is zinc sulfate.

13. The acid electroplating bath according to claim 1, wherein the zinc salt is zinc acetate.

14. The acid electroplating bath according to claim 1, wherein the condensation product of a naphthalene sulfonic acid and formaldehyde is present in the bath in an amount between about 0.2 g./l. and 20 g./l.

15. The acid electroplating bath according to claim 1 containing, in addition, dimethyl formamide in an amount between about 5 ml./l. and about ml./l.

16. The acid electroplating bath according to claim 1, wherein the primary amine is an aromatic amine.

17. The acid electroplating bath according to claim 1, wherein the condensation product is the sodium salt of methylene bis-(naphthalene sulfonic acid).

18. In a process of electroplating articles and depositing thereon a bright zinc deposit, the improvement wherein said articles are electroplated in a zinc bath comprising an aqueous acidic solution of a zinc salt, a primary amine selected from the group consisting of a primary monoamine with at least four carbon atoms in its molecule and a polyamine with at least four carbon atoms in its molecule, and a water soluble condensation product of a naphthalene sulfonic acid or salt thereof with formaldehyde, said condensation product containing at least two naphthalene sulfonic acid radicals connected (with each other by methylene bridges, the primary amine being present in the bath in an amount of at least 0.5 g./l. and the water soluble reaction product being present in an amount of at least 0.2 g./l.

19. The process according to claim 18, wherein the primary amine is an aliphatic amine.

20. The process according to claim 18, wherein the primary amine is an aromatic amine.

21. The process according to claim 18, wherein the condensation product is the sodium salt of methylene bis- (naphthalene sulfonic acid).

References Cited UNITED STATES PATENTS 905,837 12/1908 Broadwell 204 -55 2,245,086 6/ 1941 Bray et al 20455 2,355,505 8/1944 Bray et al. 20455 2,384,300 9/1945 Harford 204-55 2,393,741 1/1946 Bray et a1 20455 2,543,545 2/1951 Faust et al 204-55 3,219,559 11/1965 Ostrow et al 20449 G. L. KAPLAN, Primary Examiner m UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,537,959 Dated November 3, 1970 lnventol-(s) Joachim Korpiun and Hans Joachim Steeg It is certified that error appears in the above-identified patent and that said Letters Patent; are hereby corrected as shown below:

Column 1, line &2, "a elecroplating" should read an electroplating Column 2, line '1, "ploting" should read plating line 19, "olso" should read also line 21, "requries" should read requires line 55, "polymamines" should read polyamines Column 3, line 39, "crease" should read creases Column I, line 60, before "hydrochloric" sulfuric or and before "added" are should be added.

wdiu'ii) SEALED JAN '5 19 SEAL Atteat:

mm E- Edward M. Fletcher, Ir. flomissiom of PM! Attesting Officer 

